Within a feedback control system, a calculation can be made to determine the velocity of a moveable member controlled by an actuator, such as a hydraulic or electric actuator. From this velocity value, a proportional opposing force can be calculated and fed back into the control loop to effectively damp operation of the actuator. As the system velocity increases, the force command is reduced, and thereby reduces damping of the system's response.
The disadvantage of this type of feedback control within a reactive closed loop control system is that it can lead to ‘force errors’ or ‘force lag’. This is a situation where the desired force cannot be achieved as the damping calculation is reducing the commanded force to maintain a stable system. Due to this, damping functions within control systems are currently only suitable for slow response or statically loaded systems. Damping functions are unsuitable for high response, dynamic systems as the ‘force errors’ or ‘force lag’ become very high.
As an example, aircraft components such as landing gear doors are tested during development by loading a door in a test rig so as to simulate the loads experienced when the door is opened and closed in use. Typically, a hydraulic loading actuator is connected between the test rig and the door so as to apply loading to the door, and a feedback control loop varies the loading applied depending upon the position of the door. The feedback control loop comprises a load sensor that senses the load applied by the actuator and a servo valve, which controls the supply of hydraulic fluid to the actuator. If the landing gear door moves very rapidly in operation, the feedback control loop has to be highly responsive in order to maintain the required loading as the door moves. However, highly responsive systems can become unstable at low speed or during static conditions. An object of the invention is to provide an improved loading simulation test system.